U.S. Pat. No. 4,138,474 relates to a device for immunological determinations including a housing carrying sorbent material in the form of gel particles and a porous barrier enclosing the gel particles. The barrier is located upstream from the sorbent and is in the form of a nylon fabric or net having openings which do not permit passage of the gel particles. Liquid is drawn into the syringe in an amount to fill the volume below the barrier and preferably in an amount to at least cover the net.
U.S. Pat. No. 5,139,031 relates to a staged filtration device for cytology and microbiological testing. Membrane filters having different porosity are placed in stackable containers. The membrane filters are made of cellulose or polycarbonate. Cell suspension is collected on the membrane. No sorbent is involved in the filtration device.
U.S. Pat. Nos. 4,891,134 and 4,990,253 relate to a sample filtration device employing differential pressure. Filter media are disposed near one end and retained within the core of the hollow plunger by friction means. The filter media include paper, glass fiber, cellulose, and nitrocellulose, the preferred media being fiberglass combined with polypropylene. Depth filters are generally preferred over membrane filters. No sorbent is involved in the filtration device.
U.S. Pat. No. 4,923,680 relates to a device containing a time-delay filter, particularly useful in immunoassay. The filter is positioned between an upper and a lower compartment. The filter comprising the membrane has pores which retain agglutinated or complexed materials on the filter but which allow fluids and unagglutinized materials to pass through. The filter is constructed from glass, ceramics, fibers, or synthetic and natural polymers, the preferred material being cellulose esters, polyamides, and polyesters. The filter is overcoated with at least one water-soluble protein or carbohydrate.
U.S. Pat. No. 4,131,544 relates to a sorbent material encapsulated in a porous filter membrane composed of polypropylene or Teflon.RTM., also known as polytetrafluoroethlyene. The pore size of the encapsulating membrane is smaller than the particle size of the sorbent material.
U.S. Pat. No. 3,493,503 relates to a disposable and removable filter disposed downstream from the contents of a syringe to produce a protein-free fluid.
U.S. Pat. No. 4,820,276 relates to a filter assembly having compartments containing filter material.
In these devices, the sorbent material separates from the containment devices and contaminates the filtrate. The sorbent also dries out during certain processes in which it should be kept wet to obtain ultimate performance of the device.